gs-details-page: Allow fallback sizes for icon

When running on hi-DPI screens, some apps don’t have a suitable icon to
display at 96px at scale factor 2 (i.e. 192px). Rather than displaying
a generic icon, fall back to 64px at 2× scale (i.e. 128px), which all
apps have.

Spotted by Milan Crha.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>

Helps: #1147
2 jobs for 1147-icons-refactoring in 7 minutes (queued for 1 second)
Status Name Job ID Coverage
  Build
passed debian-stable-x86_64 #1197792

00:03:52

passed fedora-x86_64 #1197791

00:07:00